Container aware migrations

This would allow much more powerful migrations as right now all you can do is SQL stuff.

Moved from https://github.com/symfony/DoctrineMigrationsBundle/pull/2
"Using $this when not in object context"
Updated the composer.json
Merge pull request #28 from frosas/patch-1

Container aware migrations
Updated the composer constraint
Container aware migrations

Added section for container aware migrations
Merge pull request #50 from peterrehm/patch-1

Container aware migrations
Update composer.json
Merge pull request #52 from totaltrash/master

Updated composer.json for Symfony 2.3
Merge